Medium voltage (MV) transformers step up DC/AC inverter output voltage to allow connection to an MV distribution grid. Transformer power losses consist of a) winding losses due to transformer load current and b) winding and core losses due to transformer magnetizing current.
The transformer power losses can be categorized as i) load losses during transformer operation (i.e. inverter power>0) or ii) no-load losses occurring when the transformer is idle (inverter power=0).
Studies show that the no-load losses account for up to 75% of the total transformer power losses depending on configuration and should thus preferably be eliminated or at least reduced. Consequently, the MV transformer should be disconnected when there is no power generation in the power generating means delivering a DC voltage to the inverter, which power generating means for example may be embodied in the form of a photovoltaic (PV) array.
However, connection of the transformer to the distribution grid results in an inrush current flow in the primary winding, which results in strong magnetic fields and misplacement of the windings. Consequently, insulation properties are deteriorated. Similar effects also occur during disconnection since the magnetizing current is forced to stop flowing, and frequent connection/disconnection hence reduces transformer reliability and lifetime.
In the art, a series choke has been used to limit the inrush current. However, this requires a bulky additional component the design of which has to be adapted to the specific transformer requirements.